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<title>WebGL - Multiple Views clear issue</title>
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Multiple Views clear issue
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for most samples webgl-utils only provides shader compiling/linking and
canvas resizing because why clutter the examples with code that's the same in every sample.
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and https://webglfundamentals.org/webgl/lessons/webgl-resizing-the-canvas.html
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<script src="resources/webgl-utils.js"></script>
<script src="resources/m4.js"></script>
<script src="resources/primitives.js"></script>
<!-- vertex shader -->
<script id="vertex-shader-3d" type="x-shader/x-vertex">
attribute vec4 a_position;
attribute vec4 a_color;

uniform mat4 u_matrix;

varying vec4 v_color;

void main() {
  // Multiply the position by the matrix.
  gl_Position = u_matrix * a_position;

  // Pass the vertex color to the fragment shader.
  v_color = a_color;
}
</script>
<!-- fragment shader -->
<script id="fragment-shader-3d" type="x-shader/x-fragment">
precision mediump float;

// Passed in from the vertex shader.
varying vec4 v_color;

void main() {
  gl_FragColor = v_color;
}
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<script>
'use strict';

function main() {
  // Get A WebGL context
  /** @type {HTMLCanvasElement} */
  const canvas = document.querySelector('#canvas');
  const gl = canvas.getContext('webgl');
  if (!gl) {
    return;
  }

  // setup GLSL programs
  // compiles shaders, links program, looks up locations
  const programInfo = webglUtils.createProgramInfo(gl, ['vertex-shader-3d', 'fragment-shader-3d']);

  // create buffers and fill with data for a 3D 'F'
  const bufferInfo = primitives.create3DFBufferInfo(gl);

  function degToRad(d) {
    return d * Math.PI / 180;
  }

  const settings = {
    rotation: 150,  // in degrees
  };
  webglLessonsUI.setupUI(document.querySelector('#ui'), settings, [
    { type: 'slider',   key: 'rotation',   min: 0, max: 360, change: render, precision: 2, step: 0.001, },
  ]);

  const fieldOfViewRadians = degToRad(120);

  function drawScene(projectionMatrix, cameraMatrix, worldMatrix) {
    // Clear the canvas AND the depth buffer.
    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

    // Make a view matrix from the camera matrix.
    const viewMatrix = m4.inverse(cameraMatrix);

    let mat = m4.multiply(projectionMatrix, viewMatrix);
    mat = m4.multiply(mat, worldMatrix);

    gl.useProgram(programInfo.program);

    // ------ Draw the F --------

    // Setup all the needed attributes.
    webglUtils.setBuffersAndAttributes(gl, programInfo, bufferInfo);

    // Set the uniforms
    webglUtils.setUniforms(programInfo, {
      u_matrix: mat,
    });

    webglUtils.drawBufferInfo(gl, bufferInfo);
  }

  function render() {
    webglUtils.resizeCanvasToDisplaySize(gl.canvas);

    gl.enable(gl.CULL_FACE);
    gl.enable(gl.DEPTH_TEST);

    // we're going to split the view in 2
    const effectiveWidth = gl.canvas.clientWidth / 2;
    const aspect = effectiveWidth / gl.canvas.clientHeight;
    const near = 1;
    const far = 2000;

    // Compute a perspective projection matrix
    const perspectiveProjectionMatrix =
        m4.perspective(fieldOfViewRadians, aspect, near, far);

    // Compute an orthographic projection matrix
    const halfHeightUnits = 120;
    const orthographicProjectionMatrix = m4.orthographic(
        -halfHeightUnits * aspect,  // left
         halfHeightUnits * aspect,  // right
        -halfHeightUnits,           // bottom
         halfHeightUnits,           // top
         -75,                       // near
         2000);                     // far


    // Compute the camera's matrix using look at.
    const cameraPosition = [0, 0, -75];
    const target = [0, 0, 0];
    const up = [0, 1, 0];
    const cameraMatrix = m4.lookAt(cameraPosition, target, up);

    let worldMatrix = m4.yRotation(degToRad(settings.rotation));
    worldMatrix = m4.xRotate(worldMatrix, degToRad(settings.rotation));
    // center the 'F' around its origin
    worldMatrix = m4.translate(worldMatrix, -35, -75, -5);

    const {width, height} = gl.canvas;
    const leftWidth = width / 2 | 0;

    // draw on the left with orthographic camera
    gl.viewport(0, 0, leftWidth, height);
    gl.clearColor(1, 0, 0, 1);  // red

    drawScene(orthographicProjectionMatrix, cameraMatrix, worldMatrix);

    // draw on the right with perspective camera
    const rightWidth = width - leftWidth;
    gl.viewport(leftWidth, 0, rightWidth, height);
    gl.clearColor(0, 0, 1, 1);  // blue

    drawScene(perspectiveProjectionMatrix, cameraMatrix, worldMatrix);
  }
  render();
}

main();
</script>
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